A 71-year-old man in whom a gastric submucosal lesion was found incidentally was referred to our hospital for detailed examination. Esophagastroduodenoscopy showed a submucosal lesion in the body of the stomach. Endoscopic ultrasound revealed a 15-mm hypoechoic round mass with calcifications arising from the muscular layer. Confusing the diagnosis, it resembled a gastrointestinal mesenchymal tumor. Subsequently, endoscopic ultrasound-guided fine-needle aspiration was conducted for definitive diagnosis. Pathologic analysis showed a granuloma. Because this patient had no prior exposure to tuberculosis or Helicobacter pylori infection and had no abnormal laboratory data, this submucosal lesion was diagnosed as idiopathic granulomatous gastritis.